ADCs and Pressure Transducers

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#1

Background: I’m a Mechanical Engineering senior and I am looking for help on my senior design project.

Project: surface a breath hold diver (free diver) after a timer expires with an inflatable life jacket.

Parts I plan on using:
1x Cerberus mainboard
1x Character display module
1x UC Battery 4xAA module
1x Relay X1 Module
2x Button Modules [em]Probably going to need to change this since buttons push themselves at depth[/em]
1x Elexol 8 Channel Analog to Digital I/O board (http://www.robotshop.com/elexol-8-channel-analog-to-digital-i-o-board-4.html)
1x Electrically operated valve (http://www.ascovalve.com/Common/PDFFiles/Product/Mag_LatchR1.pdf)
1x [em]Pressure Transducer[/em]

I found this pressure transducer, http://www.dwyer-inst.com/Product/Pressure/SinglePressure/Transmitters/Series628CR/Ordering which @ Gene said would work just fine.

Is the only difference between a transducer that outputs voltage vice current simply a resistor? IE could I use the proposed transducer and simply add a resistor to drop the current over it and input that into the ADC?

#2

Hi Keith,

Attached is a copy of the analog section I use to read 4-20mA sensors.

I use a 100R resistor so that the span is 0.4 to 2.0 volts as my ADC is max 2.048 volts input. I have 4 inputs in this design.

You can chose a different value of resistor to get your input range. 150 will be ideal for a 3.3V ADC

Max Current * Resistor Value = Max Volts

Eg. 0.020 * 150 = 3.0 volts

Same formula applies for minimum volts.

By the way, you need to look at the valve choice again. With your batteries in series providing approx 6V this means your valve will have to be rated to this voltage. At 33W input, that is 5.5amps of current. Better make sure your battery can handle this current output. Most AA are around 2500mAH but the voltage drops when you try to draw a high current. I read somewhere that Alkaline batteries are not good for high current loads so do some checks on this.

#3

By the way, why the additional ADC board?

The Cerberus has a 12 bit ADC so you can use that to connect your pressure sensor to. This has a better resolution than the 10 bit one you have listed. 4096 bits instead of 1024.

#4

I thought I needed an ADC because I didn’t know the Cerberus board came with one.

I surmised that it didn’t have one because it wasn’t mentioned anywhere at this link:
https://www.ghielectronics.com/catalog/product/349

How should I shop/research in the future so that an oversight like this can be avoided?

Thank you for your time,
Keith

#5

I think you need glasses Keith, :slight_smile:

It states…(I copied this from midway down the list)

2 UART
CAN (preliminary managed drivers only)
9 Analog Inputs. <---------------------------------------------
2 Analog Output

Also, the image shows sockets 2, 3 and 4 as having 3 analog inputs each.

Regards
Dave…

#6

LoL .I saw that but I didn’t know what it meant. Analog input… to what? Where? How?

The answer is analog input to ADC to microcontroller.

What I’m gathering here is that if any microcontroller says “analog input” that it’s understood that the analog input is then translated into a digital signal through an ADC.

I was actually an electronics technician for the navy and the ship I was on was ALL ANALOG. So when I read “analog input” I figured it would be an input to some other sort of device that required an analog signal.

So… I guess now that I’m aware that the microcontroler has an ADC built into it, I suppose that connecting to it would be a matter of selecting the appropriate socket and connecting a “Breakout Module?”

#7

Correct – The socket you are looking for is an “A”.

Pins 3, 4 & 5 are the ones connected to the ADC.

Here’s a link to the big picture as far as socket labels and their respective meanings